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Ceres surprises with water ice and colourful bright spots

NASA/JPL-Caltech/UCLA/MPS/DLR/IDA/PSI/LPI

By Jacob Aron in The Woodlands, Texas

Ceres, the largest asteroid in the solar system, is continuing to reveal its secrets. The latest data from NASA’s Dawn probe, which has been in orbit around the dwarf planet since April last year, suggests this tiny world has water ice on the surface.

Dawn has been gradually lowering its altitude to get an ever closer look at this mysterious world. Today at the Lunar and Planetary Science Conference in The Woodlands, Texas, the mission team unveiled their latest findings, including a spectacular view of the beguiling bright spots at the heart of the 92-kilometre-wide Occator crater (above) from the lowest orbit of just 375 kilometres up.

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These spots have perplexed researchers since Dawn first reached Ceres, and we think probing their nature will provide clues to Ceres’s interior. In the new picture, Dawn has seen colour variations across the surface of the bright regions. These wouldn’t show up to the human eye, but reflect possible differences in the composition of the material seen by Dawn, said Ralf Jaumann of the German Aerospace Center in Berlin. “The big questions is the reason for this colour difference.”

This close-in view reveals that the central bright spot actually sits within a depression inside the crater, about 10 kilometres across, and at the centre of that is a small mound. “We’re starting to see how complex the distribution of the bright material is,” said Carol Raymond of NASA’s Jet Propulsion Laboratory in Pasadena, California.

Just how this arrangement formed is still a mystery. Timothy Bowling of the University of Chicago presented a model of a possible explanation, in which a meteorite smacks into the surface of Ceres, exposing icy material from as much as 40 kilometres below the surface and heating it up. “Occator is probing deep into Ceres sub-surface,” he said. As this material settled into the Occator crater we see today, the water would evaporate, leaving bright salt and minerals behind.

The floor of Occator is also riddled with fractures that seem to be older than the crater itself. These could have provided a handy route for material beneath the surface to be squeezed out when the crater was formed.

Ice layer

The team has also made an unexpected discovery: water ice hiding in a crater. The crater, called Oxo, is a 10 kilometre-wide impact that seems to have occurred relatively recently – between 1 and 10 million years ago, based on the size and frequency of other craters in the region. The find comes from spectral data taken during Dawn’s higher orbit in June last year.

Seeing water ice anywhere on Ceres is a surprise as the surface is generally warm enough that any ice should evaporate into space. That means it must have been exposed recently, said Jean-Philippe Combe of the Bear Fight Institute in Winthrop, Washington. “This area is possibly a cold trap where H20-rich materials could be preserved for at least some time,” he said.

The water is near the side of the crater and could be in one of two possible forms: either regular ice, or locked up inside hydrated minerals. The spectral data fits best with a model of water ice, but the exact form isn’t yet clear.

So where has this water come from? Models of Ceres’s formation, along with the sightings of bright spots like the ones at Occator, suggest the dwarf planet has an icy sub-surface layer that is mixed up with salt and rock. Combe believes that the ice at Oxo could have been exposed by a landslide, or dug up by a meteorite impacting the surface.

Normally on Ceres this ice would boil away, leaving behind bright salts, as is thought to be the case at Occator. But Oxo provides a cool place for ice to hang on. “All of this occurs in a location that is shadowed most of the time,” said Combe.

It’s also possible that the location of Oxo could line up with a distant sighting of water vapour coming from Ceres made by the Herschel space telescope in 2014, said Kynan Hughson of the University of California, Los Angeles. Repeat observations of Oxo will let the Dawn team watch for any changes in the ice, which would suggest the ice there is evaporating into space.

Dawn will continue to gather data about Ceres until at least August, and possibly into next year, depending on the health of its reaction wheels, which are used to control its orientation. But some questions about this enigmatic asteroid may not be answered until a repeat visit. “It would be nice to land there, wouldn’t it?” said Raymond.